End pleats are the two outer pleats on opposing end faces of the filter bellows of a filter element. End edges are the two free edges of the filter medium from which the filter bellows is made, which run along the end pleats and confine these at the end faces of the filter bellows. End edges of the filter bellows are the two other free edges of the filter bellows, which extend between the end edges and run along the folding of the filter bellows. The fold edges are the edges along which the filter medium is folded. In a zig-zag folded, roughly rectangular-shaped filter bellows known from the market the end edges and the fold edges usually run parallel to one another. The end edges viewed from the side to the filter bellows run zig-zag-shaped and perpendicular to the end edges and fold edges. Before folding of the filter medium, the end edges of the later roughly rectangular-shaped imagined enveloping filter bellows run straight and parallel to one another. The imagined enveloping is clamped by the end edges, the fold edges adjacent on a clean fluid side of the filter bellows neighboring fold edges or end edges and on a raw fluid side neighboring fold edges or end edges.
The filter media for filter bellows of flat filter elements are not closed, which means that the end pleats are not connected with each other just as the end edges. In contrast, the filter media for filter bellows of round filter elements are closed, which means that the end pleats are connected with each other. Filter bellows of flat filter elements can be level but also have bends in different directions.